In the fabrication of semiconductor devices, connections to the semiconductor device are typically accomplished by metallic connection points or bond pads (I/O pads) that are disposed on a planar surface of the device (or die) around the periphery thereof. Once the functionality and the circuit requirements of a semiconductor device are met in the design, the required number of bond pads for operating the device can be determined. The bond pads are normally positioned on a semiconductor device in a peripheral area which is a ring-shaped area on the surface of the device or a narrow band between the edges of the device and an interior area of the device. One of the reasons that bond pads are disposed around the edges of a device is that the peripheral location permits a relatively large number of I/O connections to the die without causing the connections to cross one another.
Bond pads are formed in a metal conductor layer deposited and then patterned on the top surface of a semiconductor device. The metal conductor layer is frequently embedded in an insulating layer of a dielectric material. In order to establish electrical communication with the bond pads, a contact plug is normally used which is formed of a conductive metal material such as tungsten or aluminum.
The bond pad openings (or contact openings), when filled with an appropriate conductive material form void-free contact plugs and exhibit low contact resistance to the underlying and overlying conductors. Other than metallic materials such as tungsten and aluminum, heavily doped polysilicon can also be used in contact plugs. For instance, polysilicon can be doped n-type when contacting N-regions and p-type for P-regions to avoid inter-diffuision and dopant migration. When a metallic material such as tungsten or aluminum is used to fill a contact window, the window is typically lined with a thin layer of titanium or titanium/titanium nitride (TiN) prior to the contact plug fill. Other similar compositions such as titanium tungsten and tungsten can also be used. The main purpose of titanium is to improve the contact resistance. The TiN film is deposited to act as a diffusion barrier to certain elements such as silicon from the substrate and fluorine generated during a tungsten chemical vapor deposition process. The thin layer of Ti or TiN also act as a glue layer to improve adhesion to tungsten. The layers may further act as a wetting film to enhance the reflow of aluminum. The liner of Ti and TiN are typically deposited by a collimated sputtering process or a chemical vapor deposition process. A desirable thickness for the Ti layer is between about 200 .ANG. and about 800 .ANG., while the same for the TiN layer is between about 800 .ANG. and about 2500 .ANG..
Contact hole openings can be filled by depositing tungsten in a chemical vapor deposition or a sputtering process and then planarizing the metal by etching it back to the insulator surface or by a chemical mechanical polishing process such that only tungsten is left in the contact openings. Tungsten CVD is used for contact hole filling and is typically performed by the pyrolitic decomposition of tungsten hexafluoride (WF.sub.6), or by the reduction of WF.sub.6 with hydrogen, silicon or silane. In semiconductor manufacturing, reduction WF.sub.6 at a temperature between 250.degree. C..about.600.degree. C. is more frequently used.
In the tungsten plug process, TiN is used as a barrier layer and a glue layer. However, TiN grains have a columnar structure which allows fluorine from the reaction precursor to easily migrate through the crevices of TiN to react with an underlying titanium layer during the tungsten deposition process. The fluorine causes the peeling of the glue layer TiN and the tungsten layer deposited. A conventional tungsten plug formation that has the volcano defect is shown in FIG. 1.
FIG. 1 illustrates a semiconductor device that is built on a semi-conducting substrate 12. A contact hole 14 is opened through an inter-level dielectric layer 16 to provide communication with the source/drain active region 20. Inside the contact hole and on top of the inter-level dielectric layer 16, barrier layers of titanium 22 and titanium nitride 24 are deposited. A layer of conductive metal such as tungsten 32 is then deposited bya chemical vapor deposition (CVD) process over the surface of the substrate 12 to fill the contact hole 14.
During the tungsten CVD process, fluorine from the reactant precursor WF.sub.6 migrate through the TiN film 24 to react with the titanium layer 22 and cause the peeling of the tungsten and the TiN layer from the substrate 12. This is shown in FIG. 1. The volcano effect, shown as 30 in FIG. 1, reflects the delamination of the tungsten and titanium nitride layers from the substrate 12. The two layers are separated or peeled away from the substrate 12. It is believed that during tungsten CVD WF.sub.6 reacts with the underlying layer of Ti to produce W and TiF.sub.3. Since TiF.sub.3 is a material that has poor adhesion to tungsten, it does not adhere to and peels away from the tungsten layer 22. A plane view of a semiconductor structure having typical bond pad openings is illustrated in FIG. 2, showing the openings are formed by straight line borders. The black dots shown in FIG. 2 along the bond pad openings are the volcano defects.
Others have attempted to improve the tungsten plug process and to avoid the volcano defect. One such method is an annealing process of a rapid thermal annealing method to improve the density and thus to reduce voids in a TiN film. The theory is that oxygen or nitrogen products can be used to stuff the TiN grain boundaries such that fluorine cannot penetrate through. However, the annealing method is a time-consuming process which creates a bottle neck in the total semiconductor fabrication process.
It is therefore an object of the present invention to provide a method for forming bond pad windows in a semiconductor structure that does not have the drawbacks and shortcomings of the conventional tungsten plug process.
It is another object of the present invention to provide a method for forming bond pad windows in a semiconductor structure that provides windows of reduced stress in the underlying TiN barrier layer without the formation of volcano defect.
It is a further object of the present invention to provide a method for forming bond pad windows in a semiconductor structure that does not create straight-line border bond pad openings.
It is another further object of the present invention to provide a method for forming zig-zag bordered opening in a semiconductor structure that creates openings of substantially reduced stress in an underlying TiN barrier layer.
It is still another object of the present invention to provide a method for forming zig-zag bordered openings in a semiconductor structure that creates openings having saw-tooth configuration in the borders of the openings.
It is yet another object of the present invention to provide a method for forming zig-zag bordered bond pad openings in a semiconductor structure wherein the pitch of the zig-zag border created between about 2 micron and about 6 micron.